1. Field of the Invention
The present invention relates to a method for producing a molded article in which a resin part is injection-molded to an opening of a main body part. The invention is also directed to a molded article of the type having a resin part injection-molded to an opening of a main body part.
2. Description of the Related Art
A connector case used in, for example, electric components or automobile parts, has a main body made of metal or the like and a cable connecting part that is frequently molded from a resin. Such a connector case is produced by molding the connecting part in such a way that the resin is injection-molded to an opening provided on the main body part. Apart from the connector case exemplified above, widespread use is made of a method for molding a resin part by injection-molding a resin to an opening of a main body part.
In order to prevent water or the like from entering the main body part, the junction area (hereinbelow, often referred to as a “surface boundary”) between the main body part and the resin part must be airtight. Conventionally, to achieve the required air-tightness, a sealant such as an O-ring or the like is first adhesively attached to the peripheral portion around the opening of the main body part, after which the resin part is injection-molded. However, since conventionally used sealants such as the O-ring exhibit high repulsion and low compressibility, it is impossible to perform the injection molding of the resin part with the sealants sufficiently compressed. Thus, due to the shrinkage occurring during cooling and solidification of the molten resin, a gap may be created at the surface boundary between the resin part and the sealant or between the sealant and the main body part, thus detrimentally affecting the air-tightness between the resin part and the main body part.
Japanese Patent Application Publication No. JP-A-10-15985 describes a technique for injection-molding a resin part to a main body part, wherein a thermally expandable material is attached beforehand around the target portion of the main body part to which the resin part is to be injection-molded, so that the air-tightness is attained between the main body part and the resin part due to the expansion of the thermally expanded material by the heat of the molten resin during the injection molding.
In the described technique, the sealant containing a thermally expandable material is secured around the target portion of the main body part to which the resin part is to be injection-molded. The main body part is placed within a mold and, subsequently, the hot molten resin is injection-molded into the mold. The heat of the molten resin causes the thermally expandable material to expand.
The expansion of the thermally expandable material fills minute gaps existing between the main body part and the resin part, thereby assuring an increased air-tightness. Even if the molten resin is cooled down and shrinks, the sealant expands proportional to the shrinkage, thus providing an advantage in ensuring the required air-tightness at the surface boundary between the main body part and the resin part.
However, in the described technique, the expansion of the thermally expandable material may sometimes be insufficiently in the case where the thermally expandable sealant is attached around the target portion of the main body part to which the resin part is to be injection-molded and then the thermally expandable sealant is expanded by the heat of the molten resin in the injection molding process. For example, if the resin part to be molded has a relatively small volume, the molten resin may fail to carry enough heat to sufficiently expand the sealant. Insufficient expansion of the sealant makes it impossible to attain the required air-tightness.
To resolve this problem, Japanese Patent Application Publication No. JP-A-10-15985 suggests reheating the molded article after the injection molding process, which allows the sealant to expand after the molding process of the resin part is completed, thereby ensuring the required air-tightness. However, this technique requires an additional processing step of reheating the molded article.
Demand exists for an injection molding technique that ensures the required air-tightness at the surface boundary between the main body part and the resin part without requiring an additional step of, e.g., reheating the molded article.